In recent years, there has been a tendency for the material used to form cylinder blocks to shift from cast iron to aluminum in order to reduce the weight of engines, and cast iron produced by a centrifugal casting process has been increasingly employed as cylinder liners in the blocks.
Centrifugally cast iron has low machinability compared to common cast iron produced using sand molds, and the life of tools used for cutting centrifugally cast iron is noticeably short, which is a problem.
Since centrifugally cast iron has fine, dense A-type structures, which are also observed in conventional sand mold cast iron, wear of a cutting tool is significantly increased by thermal reaction with a workpiece, resulting in a marked decrease in machinability of centrifugally cast iron.
It is known that a sintered cBN compact to which Al2O3 having excellent oxidation resistance and chemical stability has been added is effective as a cutting tool in order to machine such centrifugally cast iron having a property of being difficult to machine. However, the toughness and sinterability of a sintered cBN compact to which Al2O3 has been added are low. In order to overcome this problem, Patent Literatures 1 and 2 each disclose a sintered cBN compact whose fracture resistance is improved by addition of ZrO2 to Al2O3.
For example, Patent Literature 1 discloses a sintered cBN compact including Al2O3, TiC or TiCN, and ZrO2, in which the sintered cBN compact is produced using a starting material containing 50% by volume or more and 80% by volume or less of the cBN component, 1% by volume or more and 20% by volume or less of TiC or 0.5% by volume or more and 15% by volume or less of TiCN, and 15% by volume or more and 50% by volume or less of Al2O3 and ZrO2, the ZrO2/Al2O3 ratio by weight being 0.1 or more and 4 or less. When the ratio by weight specified by ZrO2/Al2O3 is converted to a ratio by volume, 0.065≦ZrO2/Al2O3≦2.62.
Furthermore, Patent Literature 2 discloses a material for a sintered compact having a composition including 40% to 70% by volume of cBN powder particles, 15% to 45% by volume of titanium nitride serving as a major component of a binder phase and 15% to 35% by volume of mixed powder particles of Al2O3, ZrO2, AlN, and acicular crystals of SiC serving as an accessory component of the binder phase, in which the accessory component of the binder phase is composed of 50% to 65% by volume of Al2O3, 1% to 5% by volume of ZrO2, 20% to 40% by volume of AlN, and 5% to 15% by volume of acicular crystals of SiC.
In a centrifugal casting process, by pouring molten cast iron into a rotating cylindrical mold, it is possible to produce a thin sleeve, which is advantageous. However, since a portion of the sleeve in contact with the mold and a portion of the sleeve located at the inner diameter surface are rapidly cooled, the cast iron structure is refined, resulting in generation of abnormal structures, such as dendritic structures and rosette structures. These abnormal structures, such as dendritic structures and rosette structures, have very poor machinability. In most recent sleeves made of centrifugally cast iron, the thickness has been decreased in order to shorten the distance between cylinder bores, and it has become necessary to cut, in addition to fine, dense A-type structures which are regions to be cut in conventional centrifugally cast iron, regions where rosette structures and dendritic structures exist, which are not regions to be cut in conventional centrifugally cast iron. When regions where rosette structures and dendritic structures exist are subjected to cutting, machining is required to be carried out under thermally harsh conditions compared with conventional centrifugally cast iron, and therefore, machinability is markedly decreased, which is a problem.
When such centrifugally cast iron is machined using the sintered compact described in any of the above-mentioned Patent Literatures, since a large amount of ZrO2 having very low thermal conductivity compared to Al2O3 is contained, the thermal conductivity of the sintered compact decreases, and reaction with the workpiece under machining proceeds markedly, resulting in a significant decrease in wear resistance.